1. Field of the Invention
This invention relates to a method of fabricating an article made of a composite material by providing a compound material, producing a green body by forming and compacting the compound material into a desired compact form, producing the article by curing a curable matrix-forming binder in said green body, and, optionally, post-curing the matrix; and further relates to apparatus for use in carrying out the method, and articles of manufacture fabricated by the method.
Articles of manufacture of composite materials according to the present invention are useful in a multitude of applications particularly where in use the article must retain its shape and properties when subjected to temperatures above the melting point of usual thermoplastic materials, e.g. above 1000.degree. C. Typical examples of articles include friction pads, commutators, rubber parts, heat shields, electrical insulations, connectors, machinery parts, and magnets and bearings.
2. Prior Art Disclosure
GB Patent Application No. 2 012 204 discloses a composite friction assembly comprising at least one friction member and a support member, said friction member being formed from a mixture of a thermosettable phenol-aldehyde resin and filler material by heating a mould to a temperature of the resin, usually between about 160.degree. C. and about 180.degree. C., and exerting on the mixture a pressure, normally between about 2068 bars (approximately 30,000 psi) and about 3447 bars (approximately 50,000 psi) in a time between about 2 second and 10 second.
EP Patent Application No. 0 352 363 discloses an acrylic containing friction element manufactured by compressing a mixture comprising a thermosetting organic binder, a fibrous reinforcing material, and an effective amount of fibrillated acrylic polymer fibers to form a preform; pressing the preform at elevated temperatures; treating the pressed material at a temperature sufficient to effect curing and then forming a friction element such as disc brakes, friction pads, clutch linings from the cured material. The pressure to form the preform is in the range from 140 to 350 bars (approximately 2000 to 5000 psi), and the curing of the preform is accomplished by heating to a temperature in the range from 350.degree. to 600.degree. F. (175.degree.-315.degree. C.) for about 15 min., whereafter the preform is post-baked for several hours.
A disadvantage of these processes is that the compaction of the compound material has to be carried out at pressures as high as 200-500 bars, or more, for a satisfactory flow and compaction of the moulding material to occur which requires costly tooling and press equipment.
Further, for economically short forming and curing times, the cure of the curable matrix binder has to proceed at an elevated curing temperature typically in the range from about 135.degree. C. to about 250.degree. C., or more, to ensure that the curing temperature has been reached in all parts of the 30 article. Otherwise, if lower curing temperatures are applied, long forming and curing times are required which limits the production rate.
The curing time depend on the nature of the curable matrix binder and the dimensions of the article. For large articles, such as friction elements for brake pads, the curing time can be from about 15 min. to about 75 min., or more before sufficient mechanical stability Is obtained, and the article can be removed from the mould and transferred to an oven for post-cure.
Another disadvantage of these processes is that the heating is provided by conductive transfer of heat from the mould into the compound material the homogeneity of which heat transfer is difficult to control and results in a non-uniform polymerisation of the curable matrix-forming binder.
This deficiency of conductive heat transfer into moulds has been recognized, and methods of providing a homogeneous and accelerated transfer of energy-into the compound material have been disclosed.
EP Patent Application No. 0 347 299 discloses the use of capacitative microwave heating at pressures from 50 to 200 bars in moulding glass-filled unsaturated polyester resin bonded composites; and German Laid Open Patent Application No. 29 06 842 discloses the use of capacitative high frequency microwave heating with a frequency of 2450 MHZ for moulding of thermosetting materials. Both these methods are not suitable for composite materials containing significant amounts of metals. Further, they involve electrical shielding which is impractical to handle, and the apparatus is complicated.
EP Patent Application No. 0 093 655 discloses a method of fabricating an organo-metallic composite material comprising a mixture of a thermosetting resin, a metallic powder, and additives intended to confer certain properties to the final product by simultaneously application of induction heating and pressure, said pressure ensuring a good electrical conductivity of the composite material.
This method requires a high content of electric conducting material having an appreciable hysteresis loss.
GB Patent Applications Nos. 2 106 823 and 2 135 412 disclose a method of resistive heating and sintering of metallic friction materials including a method of producing a friction assembly of friction material to a support member using resistive heating. This method requires a sufficiently high proportion of electrically conducting material.
Besides the disadvantages mentioned these processes have a high probability of providing articles having void formations and non-uniform distribution of material components, particularly when the compound material comprises particulate materials of different sizes and densities and/or comprises one-or more constituents in very low concentrations.
Alternative methods of heating by application of ultra sonic energy have been disclosed.
U.S. Pat. No. 4,487,728 discloses a method of producing mouldings of specific thermoplastic materials in a multi cavity mould using ultrasonic vibration. Sintered blanks are obtained which are capable of further thermo-forming to the required shape.
U.S. Pat. No. 4,548,771 discloses a method of vulcanizing conventional rubber materials comprising 75% by volume of base elastomer, thus exclusively elastomeric In nature, and less than 25% by volume of filler material by initially applying ultrasonic energy at a maximum intensity of 31 W/cm.sup.2 and asymptotically reducing the intensity to 40-45% of its initial value in the course of 3-5 min. there by avoiding degradation of the elastomer due to exposure to excessive heat. Nothing is disclosed or suggested about curing composite materials containing a curable matrix forming binder in less than 5 to 50% by volume.
Methods of producing friction products by a two-step procedure comprising preheating of frictional material by applying ultrasonic vibrations onto the material; and then applying pressure to the preheated material have been disclosed.
WPI Acc. No. 83-26393K/11 publishes an abstract of Japanese Patent Application No. 58 020 413 (Australian Patent Application No. 86289-82) disclosing a method of moulding a friction material comprising providing a back metal on a support bed; filling particles of a friction material in the moulding cavity formed between a die and the support bed; melting the particles by heating using ultrasonic vibration; then transferring the die and support bed, the particles and the back metal to a pressing process wherein the particles-are moulded so that the moulded friction material is welded to the back metal.
An ultrasonic wave forming method comprising simultaneous application of ultrasonic wave oscillation and forming under pressure to the friction material is mentioned. However, this application teaches away from a method of frabricating an article of manufacture by a one-step process applying ultrasonic heating and pressure simultaneously, because increasing pressure allegedly results in a retardation of the speed of the ultrasonic wave oscillations.
WPI Acc. No. 84-265815/43 publishes an abstract of Japanese Patent Application No. 59 023 804 disclosing a method of producing a friction product for brakes comprising preheating a frictional powder containing inorganic, organic and/or metallic fibres, friction adjuster, filter, and organic coupler, and hot press-forming the powder.
WPI Acc. No. 85-046916/08 publishes an abstract of Japanese Patent Application No. 60 004 536 disclosing a method of moulding a frictional member comprising placing a back metal treated with an adhesive on the bottom of a mould; charging a powdered frictional material onto the back metal; preheating the frictional material by applying ultrasonic vibrations onto the powdered frictional material; and applying a high pressure to the frictional material either by means of the vibration horn of the ultrasonic vibrator, or a hot press, thereby obtaining that moisture and gasses are removed from the frictional material during preheating; venting is not required in the subsequent stage of moulding under application of high pressure to the frictional material; and moulding can be done in a short time period.
Patent Abstract of Japan Vol. 11, No. 1 (M-550) publishes an abstract of Japanese Patent Application No. 61 181 630 disclosing the use of ultrasonic energy in friction heating of sinterable powders. Materials are vibrated until their melting point is reached at which stage they sinter together at the points of contact. The method is aimed specifically at the manufacture of porous materials which do not contain a polymeric binder.